Question: $f(x) = \dfrac{ 9 }{ \sqrt{ 10 - \lvert x \rvert } }$ What is the domain of the real-valued function $f(x)$ ?
Explanation: First, we need to consider that $f(x)$ is undefined anywhere where the radicand (the expression under the radical) is less than zero. So we know that $10 - \lvert x \rvert \geq 0$ This means $\lvert x \rvert \leq 10$ , which means $-10 \leq x \leq 10$ Next, we need to consider that $f(x)$ is also undefined anywhere where the denominator is zero. So we know that $\sqrt{ 10 - \lvert x \rvert } \neq 0$ , so $\lvert x \rvert \neq 10$ This means that $x \neq 10$ and $x \neq -10$ So we have four restrictions: $x \geq -10$ $x \leq 10$ $x \neq -10$ , and $x \neq 10$ Combining these four, we know that $x > -10$ and $x < 10$ ; alternatively, that $-10 < x < 10$ Expressing this mathematically, the domain is $\{ \, x \in \RR \mid -10< x <10\, \}$.